Electronic Structure Contributions to Electron-Transfer Reactivity in Iron−Sulfur Active Sites:  2. Reduction Potentials

Abstract

This study utilizes photoelectron spectroscopy (PES) combined with theoretical methods to determine the electronic structure contributions to the large reduction potential difference between [FeCl(4)](2)(-)(,1)(-) and [Fe(SR)(4)](2)(-)(,1)(-) (DeltaE(0) approximately 1 V). Valence PES data confirm that this effect results from electronic structure differences because there is a similarly large shift in the onset of valence ionization between the two reduced species (DeltaI(vert) = 1.4 +/- 0.3 eV). Specific electronic contributions to DeltaI(vert) have been investigated and defined. Ligand field effects, which are often considered to be of great importance, contribute very little to DeltaI(vert) (DeltaE(LF) < -0.05 eV). By contrast, electronic relaxation, a factor that is often neglected in the analysis of chemical reactivity, strongly affects the valence ionization energies of both species. The larger electronic relaxation in the tetrathiolate allows it to more effectively stabilize the oxidized state and lowers its I(vert) relative to that of the chloride (DeltaE(rlx) = 0.2 eV). The largest contribution to the difference in redox potentials is the much lower effective charge () of the tetrathiolate in the reduced state, which results in a large difference in the energy of the Fe 3d manifold between the two redox couples (DeltaE(Fe)( )(3d) = 1.2 eV). This difference derives from the significantly higher covalency of the iron-thiolate bond, which decreases and significantly lowers its redox potential.

Cluster-Dependent Charge-Transfer Dynamics in Iron–Sulfur Proteins

“…The electronic dynamics observed in Pf Rd in the present study is consistent with the extensive studies conducted by Kennepohl and Solomon on the electronic structure contributions to ET in 1Fe-4S clusters from Desulfovibrio vulgaris rubredoxin ( Dv Rd) and in model complexes with a FeX4 cluster (where X can be a chloride or cysteinate thiolate). 45 – 47 They used a series of experimental and theoretical methods, including photoelectron spectroscopy, density functional methods, and model compound studies, to investigate the effect of electronic relaxation on ET, the reduction potential of 1Fe-4S clusters, and the kinetics of ET in 1Fe-4S proteins.…”

Studies of iron(ii) and iron(iii) complexes with fac-N2O, cis-N2O2 and N2O3 donor ligands: models for the 2-His 1-carboxylate motif of non-heme iron monooxygenases

“…In a detailed study of the relative reduction potentials of four-coordinate iron tetrathiolate compounds versus iron tetrachloride, it was proposed that the effective charge of the metal center in the reduced (ferrous) complex was the most important factor in determining the reduction potential. 74 Furthermore, it was postulated that the covalency of the tetrathiolate iron-ligand bonds gives rise to a more electron rich iron center and thus an approximate 1 V lower reduction potential compared to [FeCl 4 ] 2− . Similar factors may account for the relatively lower E1 2 of the compounds reported in this work compared to those of model complexes containing primarily nitrogen donor ligands (Table 1), which tend to be low-spin and have a stabilized reduced state due to metal back-bonding into empty ligand π* orbitals.…”

Computational studies of modified [Fe3S4] clusters: Why iron is optimal

“…However, due to the above-mentioned difficulties treating the Fermi correlation, no such functional exists, and approximate interpolation schemes have been introduced, in particular through the pioneering computational work [14] applying the broken-symmetry (BS) approach [15] to deduce the correct spin of iron-sulfur clusters [16,17]. On the other hand, other properties can be computed with high accuracy using DFT, including redox potentials for [Fe], [Fe 2 S 2 ], and [Fe 4 S 4 ] clusters [18][19][20] and for full proteins [21] as well as reorganization energies [22,23], transmission coefficients [24], and NMR parameters [25]. Recent, important studies of [Fe 3 S 4 ] clusters [26] will be discussed in greater detail later in this paper.…”